Are you certain that the leak is coming from the head gasket? I have the same issue, and I finally traced mine to the water outlet at the front of the head. It weeps water, that will follow the front of the head down to the little bit of the block deck that is exposed. From there, it makes its way back to underneath the number one spark plug, making it look like the head gasket is leaking.

-Tim
'51 Chevy 3104 w/'56 235, 848 Head, HEI Dizzy, Corvette Cam, Split Manifold and Dual Carter YF's